1. Field of the Invention
This invention generally relates to geologic surveying using hydrophones, and more particularly to hydrophone arrays requiring switchable hydrophones.
2. Description of the Related Art
Oil and gas exploration on the oceans of the earth for deposits of oil, gas and other valuable minerals is currently conducted using seismic techniques in which an exploration vessel imparts an acoustic wave into the body of water, typically by use of a compressed air xe2x80x9cgun.xe2x80x9d The air gun is activated to initiate an acoustic wave. The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The reflected wave travels back upwardly where it is detected by microphone or xe2x80x9chydrophonexe2x80x9d elements in a streamer towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. The streamer may be divided into a number of separate sections or xe2x80x9cmodulesxe2x80x9d that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signals from the hydrophone elements to the recording equipment (so-called xe2x80x9cacoustic dataxe2x80x9d).
Certain export regulations are placed on hydrophone assemblies manufactured for sale to foreign countries. One such regulation requires that hydrophone assemblies be non-operable below a specified depth. The typical disabling method is to use a pressure-activated normally-open switch as a disabling mechanism to short-circuit a hydrophone at the limit depth. The switch is connected in parallel across a hydrophone element. The pressure associated with a specified depth limit is calculated, and the switch is calibrated to close at the calculated pressure i.e. depth. When the normally-open switch closes at the specified depth, the associated hydrophone element is shorted, thus disabled. These depth-sensitive hydrophones are known as xe2x80x9cexportxe2x80x9d hydrophones.
Hydrophone streamers are assembled by electrically connecting hydrophones in a parallel circuit along the length of the streamer. A serious drawback exists when the export hydrophones described above are used in streamers. When one export hydrophone disabling mechanism is activated to disable one hydrophone element, all hydrophones in the streamer are likewise disabled. This effect is due to the parallel relationship between the disabling switch and the hydrophone elements, and the parallel relationship between the several hydrophones in the streamer. Since a streamer may contain hundreds of sensors spaced over many kilometers, a prematurely activated switch can result in the loss of great amounts of valuable data.
A pressure switch may activate at depths above the limit depth due to a calibration error, or a random switch failure. A resettable switch may help in some failure cases or when the sensor goes below the limit and then rises above the limit. However, many common switch failures result in permanent shorting of the normally-open switch. In these cases, the streamer must be reeled in. The specific failed switch must be found and replaced or the entire streamer must be replaced before data acquisition can resume. This procedure can be very time consuming and expensive to the exploration operations.
The present invention addresses the drawbacks described above by providing an export hydrophone streamer having individual hydrophone assemblies, wherein each hydrophone assmebly includes a sensor and a disabling mechanism that will disable that particular hydrophone assembly without affecting the operation of other hydrophone assemblies in the streamer.
In one aspect of the invention, a geologic survey apparatus is provided. The apparatus comprises a control platform, a plurality of geologic survey sensors electrically connected to the control platform, and a plurality of sensor disabling devices, each sensor disabling device being operatively associated with a corresponding one of the plurality of sensors. Each of the plurality of disabling devices is adapted to sense a limit condition. The disabling devices disable the corresponding one of the plurality of sensors without affecting other ones of the plurality of sensors.
In another aspect of the invention a hydrophone streamer towable from a marine vessel on a body of water is provided. The hydrophone streamer comprises a plurality of hydrophones electrically connected in parallel. Each hydrophone further comprises a normally-closed pressure activated switch connected in series with a hydrophone sensor. Each pressure switch is calibrated to open at a predetermined depth in the body of water thereby disabling the associated sensor without affecting operation of other hydrophones in the streamer.